When a woman who owns cats finds out she is pregnant, she will probably be warned to stop cleaning out the litter box. This is because cat feces can harbor a parasite called Toxoplasma gondii, which can cross the placenta and infect an unborn fetus. The infection can increase the risk of premature birth and low birth weight, and is associated with anemia, deafness, hydrocephalus, and mental retardation in the child after birth. Sometimes, if these problems aren't apparent at birth, they can develop later in life.

T. Gondii is not only found in cat owners, however. Up to a ⅓ of the population in the developed world might harbor the parasite. And for a parasite that seems to possess some potent mind control capabilities, that’s a scary thought.

T. Gondii is a protozoan parasite that likes to live inside the intestines of cats. In the world of parasites, the host where the parasite is able to thrive and reproduce is known as its definitive host; cats are the definitive host for T. Gondii. T. Gondii infection in American cats has been found to range from 16-80%, and worldwide it’s thought to be around the same as what is seen in humans: 30-40%.

Often parasites need help getting into their definitive host, and so they will utilize what is known as an intermediate host to get that help. Cats infected with T. Gondii will shed oocysts (sort of a protozoan parasite version of an embryo) in their feces. The parasite can then infect other cats if they come in contact with an infected cat’s feces.

But rodents also often come in contact with cat feces. In fact, they have a habit of digging undigested food out of cat and dog feces to make a sort of recycled dinner for themselves. Thus, they often end up ingesting T. Gondii, and that’s where things start to get interesting.

It is believed that T. gondii has evolved a way to manipulate the behavior of the rodents that it infects. After being ingested, T. gondii makes its way to their brains. Then, through a mechanism that isn’t yet understood, it does something remarkable.

Rats are born afraid of the smell of cat urine. Even if you take a laboratory rat that has never been outside of his cage--a rat that comes from hundreds of generations of other laboratory rats that never saw or smelled a cat before--and expose him to cat urine, he will exhibit a defensive and aversive response.

T. gondii, however, has found a way to influence the behavior of a rat so as to essentially erase that inborn fear. T. gondii-infected rats are much less averse to cat urine, and don’t avoid it any more than they do the urine of other rats. This change in behavior doesn’t appear to be due to a loss of smell, and it seems to be specific for cat urine.

How does this work in T. gondii’s favor? A rat that is not afraid of cat urine is more likely to put itself in situations where a cat might be nearby. Thus, it is more likely to be eaten by a cat (one of its natural predators), allowing T. gondii to end up right where it wants to be: in a cat’s intestines. Many thus believe that this is an example of T. gondii manipulating rat behavior to find its way back into its definitive host.

Rats, however, are perhaps not the only mammals whose behavior can be manipulated by T. gondii. Mounting evidence suggests that T. gondii may also affect human behavior.

How and why T. Gondii would influence human behavior is unclear. It’s possible that T. Gondii just treats rodent and human brains similarly, and any effect on human behavior is simply an unintentional byproduct of the similarities our brains have with other mammals. We have just begun to scratch the surface of T. Gondii’s effects on humans, however. If this infection truly is influencing human behavior, then perhaps we have to begin considering microbial influence as a significant factor in behavior, especially abnormal psychiatric behavior.

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